In an automatic focusing camera, positioning amount of a lens for focus adjustment is generally calculated on the basis of a defocusing amount, i.e., the amount of deviation of an image-forming focal point relative to a predetermined film plane to control the positioning of the lens by a motor for adjusting the focus.
With respect to a focus lens of which the whole body is positioned for adjusting the focus, in general, there is a relationship between a defocusing amount .DELTA.L and an amount .DELTA.d of positioning of the focus lens for adjusting the focus, which is approximately formularized as follows: EQU .DELTA.d=C.multidot..DELTA.L
wherein C represents a coefficient varying with a focal length of the lens. There has been proposed in the art a device for controlling movement of a focus lens in which the lens is positioned in the amount .DELTA.d obtained by the aforementioned focus-adjusting equation. For adjusting the focus of a zoom lens on the basis of a value calculated by the aforementioned equation, however, it is necessary to change the coefficient C in proportion to changes in a focal length of a zoom lens since the coefficient C varies with the set focal length.